docs: def ToInternal(self):
          """Convert wavelengths to the internal representation of angstroms.
          For internal use only."""
          self.validate_units()
          savewunits = self.waveunits
          angwave = self.waveunits.Convert(self._wavetable, 'angstrom')
          self._wavetable = angwave.copy()
          self.waveunits = savewunits